This kind of pane can easily be placed under "explorer pane" with a split bar for managing both heights.

Content of this pane should be accessible through plugin API in order to allow them to feed it with the structure which can be :

list of functions for a code file

list of classes

list of identifiers for a css file

etc..

I know there is Ctrl-R, but let's be honest about it, this is just not usable for serious development projects. IMHO this kind of overview is far more useful that minmap or perhaps I missed the real point of minmap (that I personally don't find useful at all).

+1 - found this thread from a search. I thought this might be the kind of thing Sublime already had. Shame it doesn't!

Splitting the file pane into two sounds good. Files at the bottom, outline at the top. Would be useful to me if the outline pulled in the 'code folding' levels and was nested. And you could say "Show 1 level / 2 levels / 3 levels deep in the outline pane". I'd use this all the time.